1. Field of the Invention
The present invention relates to a communication apparatus and method which perform media access control on the basis of the carrier sense information of a physical layer and the carrier sense information of a MAC layer.
2. Description of the Related Art
Media access control (MAC) is control for causing a plurality of communication apparatuses which perform communication while sharing the same media to decide how to use the media in transmitting communication data. Owing to media access control, even if two or more communication apparatuses transmit communication data by using the same media at the same time, there is less chance of the occurrence of a phenomenon (collision) in which a communication apparatus on the receiving side cannot separate communication data. Media access control is also a technique for controlling access from communication apparatuses to a media so as to minimize the chance of the occurrence of a phenomenon in which, despite the presence of communication apparatuses having transmission requests, the media is not used by any of the communication apparatuses.
In wireless communication, since it is difficult for a communication apparatus to monitor transmission data while transmitting the data, media access control (MAC) which is not premised on collision detection is required. IEEE 802.11 is a typical technical standard for wireless LANs, and uses CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance). According to CSMA/CA in IEEE 802.11, in the header of a MAC frame, a period (Duration) until the end of a sequence comprising one or more frame exchanges following the frame is set. In this period, a communication apparatus which is irrelevant to the sequence and has no transmission right waits for transmission upon determining a virtual occupied state of the media. This prevents the occurrence of collision. On the other hand, a communication apparatus which has a transmission right in this sequence recognizes that the media is not used except for a period during which the media is actually occupied. IEEE 802.11 defines that the state of a media is determined on the basis of such a combination of virtual carrier sense on a MAC layer and physical carrier sense on a physical layer, and media access control is performed on the basis of the determination.
IEEE 802.11 using CSMA/CA has increased the communication speed mainly by changing the physical layer protocol. With regard to the 2.4 GHz band, there have been changes from IEEE 802.11 (established in 1997, 2 Mbps) to IEEE 802.11b (established in 1999, 11 Mbps), and further to IEEE 802.11g (established in 2003, 54 Mbps). With regard to the 5 GHZ band, only IEEE 802.11a (established in 1999, 54 Mbps) exists as a standard. In order to develop standard specifications directed to further increase communication speeds in both the 2.4 GHz band and the 5 GHz band, IEEE 802.11 TGn (Task Group n) has already been established.
In addition, several access control techniques designed to improve QoS (Quality of Service) are known. For example, as a QoS technique of guaranteeing parameters such as a designated bandwidth and delay time, HCCA (HCF Controlled Channel Access) which is an extended scheme of a conventional polling sequence is available. According to HCCA, scheduling is performed in a polling sequence in consideration of required quality so as to guarantee parameters such as a bandwidth and delay time. Jpn. Pat. Appln. KOKAI Publication No. 2002-314546 refers to QoS in the IEEE 802.11e standard, and discloses a method of assigning priorities to communications between communication apparatuses in a wireless network.
When the same frequency band as that in the existing specifications is to be used in realizing an increase in communication speed, it is preferable to assure coexistence with communication apparatuses conforming to the existing specifications and to maintain backward compatibility. For this reason, it is basically preferable that a protocol on a MAC layer conforms to CSMA/CA matching the existing specifications. In this case, a temporal parameter associated with CSMA/CA, e.g., an IFS (Interframe Space) or backoff period needs to match that in the existing specifications.
Even if an attempt to increase the communication speed in terms of physical layer succeeds, the effective throughput of communication cannot be improved. That is, when an increase in the communication speed of the physical layer is realized, the format of a PHY frame ceases to be effective any more. An increase in overhead due to this may hinder an increase in throughput. In a PHY frame, a temporal parameter associated with CSMA/CA is permanently attached to a MAC frame. In addition, a PHY frame header is required for each MAC frame.
As a method of reducing overhead and increasing throughput, a Block Ack technique introduced in recently drafted IEEE 802.11e/draft 5.0 (enhancement of QoS in IEEE 802.11) is available. The Block Ack technique can consecutively transmit a plurality of MAC frames without any backoff, and hence can reduce the backoff amount to some degree. However, a physical layer header cannot be effectively reduced. In addition, according to aggregation introduced in initially drafted IEEE 802.11e, both the backoff amount and the physical layer header can be reduced. However, since the length of a physical layer frame containing MAC frames cannot be increased beyond about 4 kbytes under the conventional limitation on the physical layer, an improvement in efficiency is greatly limited. Even if the length of a PHY layer frame can be increased, another problem arises, i.e., a reduction in error tolerance.